1. Field of the Invention
The present invention relates generally to a data transmitting/receiving apparatus and method in a mobile communication system including an antenna array, and in particular, to a data transmitting/receiving apparatus and method in a mobile communication system having an antenna array for antenna diversity.
2. Description of the Related Art
In general, a radio channel environment has low reliability relative wired channels due to multipath fading, shadowing, propagation attenuation, time-varying noise, and interference. This is an obstacle to increasing data rate and thus many techniques have been proposed to overcome the limitations of the radio channels. Major examples are error control coding for suppressing the effects of signal distortion and noise and antenna diversity for overcoming fading.
Codes used for the error control coding are mainly memoryless codes and memory codes. The memoryless codes include a linear block code and the memory codes include a convolutional code and a turbo code. Depending on the type of error control coding employed, encoder outputs are divided into systematic bits (information bits) and parity bits. A major code used for separate output of systematic bits and parity bits is a turbo code although systematic bits and parity bits are also separately output with a systematic convolutional code. Here, the systematic bits are pure user information to be transmitted and the parity bits are bits added to compensate for errors generated during transmission at decoding. However, even an error control coded signal is not immune to burst errors in systematic bits or parity bits. The burst errors often occur on a fading channel. Interleaving, a technique for preventing burst errors, is a process of distributing defective data.
Generally, transmission bits are grouped into a transport block of a predetermined size as an encoder input unit in a higher layer. An encoder encodes a transport block and outputs systematic bits and parity bits. An interleaver interleaves the sequence of the coded bits in a predetermined rule. The interleaver output is processed appropriately according to a transmission scheme such as CDMA (Code Division Multiple Access), FDM (Frequency Division Multiplexing), or OFDM (Orthogonal Frequency Division Multiplexing). Then the resulting radio signal is transmitted through an antenna.
Antenna diversity is a technique of receiving a plurality of signals that have experienced fading individually to cope with the fading. Diversity technology includes time diversity, frequency diversity, multipath diversity, and space diversity. The time diversity is realized by combining channel encoding with interleaving. In the frequency diversity, signals transmitted with different frequencies undergo different multipath fading. The multipath diversity is achieved by discriminating multipath signals using different fading information. The space diversity is implemented using antenna arrays in a transmitter and a receiver alone or together to achieve diversity using mutually independent fading signals.
The error control coding and diversity techniques for radio channels, however, have limitations in satisfying the demands for high rate data service like Internet access and multimedia service unless frequency efficiency is increased. Therefore, mobile communication systems having antenna arrays have been studied to achieve high frequency efficiency.
An antenna array system having a plurality of antennas is included in a transmitter/receiver to increase frequency efficiency using space. Considering the limits of time and frequency domains, a higher data rate can be achieved easily using space. BLAST (Bell Lab Layered Space Time) or space division multiplexing is adopted as such an antenna array system. Since each antenna transmits independent information, antenna array systems are substantially MIMO (Multi Input Multi Output) systems.
Small correlation coefficients between channels established between transmission antennas and reception antennas lead to the increase of frequency efficiency and thus system capacity in an antenna array system. Only if the correlation coefficients are small, information transmitted from each of the transmission antennas takes different channels, so that a UE (User Equipment) can discriminate the information from each transmission antenna. In other words, a signal from each transmission antenna can be identified and channel capacity is increased, as long as it has a different space characteristic. The antenna array system is suitable for an environment where multipath signals have different space characteristics. Under a LOS (Line of Sight) environment, the antenna array system, which is also a multi-transmission/reception antenna system, is not so effective as a single transmission/reception antenna system. Therefore, the antenna array system is effective in an environment where multiple signal paths are generated due to the presence of many scatters between the transmitter and the receiver, i.e., an environment where correlation coefficients between the transmission and reception antenna channels are small and thus diversity effects can be achieved.
The use of an antenna array in the transmitter/receiver increases channel capacity. The channel capacity is determined according to whether the receiver/transmitter acquires information about channels transmitted from the transmitter to the receiver. The channel capacity is maximized when both the transmitter and receiver know the channel information and is minimized when neither of them knows the channel information. When only the receiver acquires the channel information, the channel capacity is in the middle, between the channel capacities of the above two cases. To acquire the channel information, the transmitter estimates channel condition or receives feedback information about the channel condition from the receiver. The channel information required in the antenna array system is channel responses between the transmission antennas and the reception antennas, and increases in proportion to the number of the transmission/reception antennas. Therefore, the antenna array system advantageously increases the channel capacity in proportion to the number of antennas available to the transmitter/receiver. However, if the channel information needs to be fed back, the increase of the antennas in numbers means as much feedback information. Hence, there is a need for increasing channel capacity, thereby reducing feedback information.
The above channel capacity increasing methods are applied to an HSDPA (High Speed Downlink Packet Access) mobile communication system.
FIG. 2 is a block diagram of a transmitter in an HSDPA mobile communication system. Referring to FIG. 2, the transmitter includes a tail bit generator 40, a channel encoder 42, a rate matcher 44, an interleaver 46, a modulator 48, a controller 50, a serial to parallel (S/P) converter 52, and an array of transmission/reception antennas 54, 56, 58, and 60.
The tail bit generator 40 adds tail bits to each of N transport blocks. The channel encoder 42 encodes the transport blocks received from the tail bit generator 40 at a predetermined code rate such as ½ or ⅓ by predetermined coding. The channel encoder 42 can be configured that it has a code rate ⅕ or ⅙ mother encoder and punctures or repeats the coded bits from the mother encoder to thereby support a plurality of code rates. In this case, selection of one of the code rates is important and performed in the controller 50.
The rate matcher 44 matches the rate of the coded bits to an intended rate. Rate matching is required when transport channels are to be multiplexed or the number of the coded bits output from the channel encoder 42 is different from the number of bits transmittable on a physical channel. The interleaver 46 interleaves the rate-matched bits and the modulator 48 modulates the interleaver output in a predetermined modulation scheme. The S/P converter 52 converts a serial modulation symbol sequence received from the modulator 48 to parallel sequences suitable for multi-transmission. The converted parallel sequences are transmitted through the transmission antennas 54, 56, 58, and 60.
The controller 50 controls the coding and modulation according to the current radio channel condition. In an HSDPA mobile communication system, the controller 50 adopts an AMCS (Adaptive Modulation and coding Scheme) by selectively using QPSK, 8PSK, 16QAM, and 64QAM. Though not shown in FIG. 2, a CDMA mobile communication system uses Walsh codes W for channelization and PN (Pseudo Noise) codes for identifying a transmitting Node B (BS).
The coded bits output from the channel encoder 42 can be classified into systematic bits and parity bits. The systematic bits and the parity bits differ in their influence on reception performance. If errors are generated at the same rate in the systematic bits and the parity bits, the errors of the systematic bits influence the overall performance of the mobile communication system more seriously then those of the parity bits. If the same error rate is maintained as a whole and more errors are generated in the parity bits than in the systematic bits, the receiver decodes more accurately than in the opposite case. It is because systematic bits substantially influences the decoder and parity bits are just added to compensate for errors generated during data transmission.
The interleaver 46 interleaves regardless of the priority levels of the systematic bits and the parity bits. That is, the conventional transmitter mixes the systematic bits and parity bits without discrimination and distributes them to the antennas. In this case, if the transmission antennas have different transmission capabilities and as a result, the transmission capability of a particular transmission antenna is small, errors are generated at similar rates in the systematic bits and the parity bits, which may affect the entire system performance. That is, the system performance may be deteriorated more seriously than when errors are generated only in the parity bits. Therefore, there is a need for increasing the entire system performance by decreasing the error rate of the systematic bits, taking into account the channel status of a signal transmitted from each transmission antenna.